1. Field of the Invention
The present invention relates to a printing apparatus and printing method. Particularly, the present invention relates to, for example, an inkjet printing apparatus using a full-line printhead having a printing width equal to the width of a printing medium, and a printing method.
2. Description of the Related Art
There has conventionally been known an inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) which prints an image on a printing medium using an inkjet printhead (to be referred to as a printhead hereinafter) having a plurality of ink discharge nozzles (to be referred to as nozzles hereinafter) arrayed as a plurality of printing element arrays.
There has been proposed a printing method of repeating an operation to print while moving the printhead relative to the printing medium in a direction (main scanning direction) perpendicular to the discharge nozzle array, and then moving the printing medium in the nozzle array direction (subscanning direction) by approximately 1/n (n is an integer of 2 or more) of the nozzle array length. This method is called multi-pass printing, and is popular because it can achieve high-quality printing at low cost by printing in an area corresponding to the nozzle array width of the printhead in n print scanning operations.
However, higher-speed printing is possible if the number of operations (to be referred to as printing passes hereinafter) to print while moving the printhead relative to the printing medium in the main scanning direction is reduced. For this purpose, it is desirable to print over the area of the nozzle array width in one scan, increase the length by which the printing medium moves in the subscanning direction, and print on the entire printing medium in a smaller number of printing passes.
In this case, image quality readily degrades at the boundary between printing passes due to density unevenness caused by the printing time difference and printing position imprecision, blur at the printed image edge, and the like.
Besides serial printing apparatuses which print while moving the printhead in the main scanning direction, others also exist which print using a full-line printhead in which a plurality of head chips, each having a printing element array of printing elements, are arranged in the printing element array direction. The printing width of the full-line printhead corresponds to the width of the printing medium, and a printing apparatus of this type can print by only conveying the printing medium perpendicularly to the printing element array direction. Even in this printing apparatus, however, image quality degrades at the boundary between head chips due to density unevenness caused by the printing time difference and printing position imprecision, blur at the printed image edge, and the like.
To prevent a degradation in quality, there has been proposed a method (to be referred to as “gapless connection” hereinafter) of printing without any gap between the printing passes of a serial printing apparatus or between the head chips of a full-line printing apparatus. There has also been proposed a method of distributing printing to two adjacent printing passes or head chips so as to create an overlap between printing passes or head chips and make the boundary between printing passes or head chips less conspicuous. The distribution method includes several methods such as distributed printing by a gradation mask.
The gapless connection printing method prints using either one of adjacent printing passes or head chips, so the edge of a printed image looks sharp at the boundary but density unevenness and the like readily stand out.
On the other hand, with the printing distribution method, the density difference between printing passes or head chips gradually changes within the overlapping area and is less noticeable, and the printing position shift and density unevenness at the overlapping area can be readily reduced.
In either printing method, the boundary is still a factor in quality degradation, and it is desirable to avoid connection processing as much as possible. For this purpose, there have been proposed various printing methods. For example, according to Japanese Patent Publication Laid-Open Nos. 2004-268326 and 2004-268394, an overlap is formed between the head chips of a full-line printing apparatus, and only one of the head chips prints a cluster of drawing objects (e.g., text, character string, graphics, or bitmap) which are printable by only one head chip. More specifically, Japanese Patent Publication Laid-Open No. 2004-268326 proposes a method of distribution to a plurality of head chips. According to this method, when head chips overlap each other, a drawing object printable by one head chip is determined to cause the head chip to print. Japanese Patent Publication Laid-Open No. 2004-268394 discloses correction means for correcting printing so that no unprinted portion occurs at the overlap between drawing objects when distributing printing to one head chip.
However, the following problem arises with these conventional methods of forming an overlap between the printing passes of a serial printing apparatus or between the head chips of a full-line printing apparatus and distributing printing to two adjacent printing passes or head chips. More specifically, the conventional methods can easily reduce the printing position shift and density unevenness at the overlap. However, in a case where edge sharpness is considered as a more important factor than density unevenness, like a text image, the edge blurs and the image quality degrades, compared to gapless connection due to a small printing position shift between a plurality of printing passes or head chips.
The processing target of the methods disclosed in Japanese Patent Publication Laid-Open Nos. 2004-268326 and 2004-268394 is limited to only a cluster of drawing objects (e.g., a text, character string, graphics, or bitmap) printable by only one head chip. The size of the processing target is small unless the overlap between head chips is sufficiently large, compared to the drawing object size. Thus, these methods are not applicable to most of drawing objects, and a plurality of head chips must print. This processing is complicated, prolongs the printing time, and decreases reliability because the position and size of a drawing object must be determined.
It is desirable to be able to print a high-quality image at the boundary in both a text image for which edge sharpness is important and a bitmap image for which suppression of density unevenness is important, regardless of whether or not either one of adjacent printing passes or adjacent head chips can print a drawing object.